"For a long time we have thought that bacteria make antibiotics for the same reasons that we love them because they kill other bacteria," said Elizabeth Shank, assistant professor of biology.

"However, we have also known that antibiotics can sometimes have pesky side-effects, like stimulating bio-film formation," Shank added.

The researchers have now shown that this side-effect the production of bio-films is not a side-effect after all, suggesting that bacteria may have evolved to produce antibiotics in order to produce bio-films and not only for their killing abilities.

Bio-films are communities of bacteria that form on surfaces, a phenomenon dentists usually refer to as plaque. Bio-films are everywhere.

In many cases, bio films can be beneficial, such as when they protect plant roots from pathogens. But they can also harm, for instance when they form on medical catheters or feeding tubes in patients, causing disease.

The study appeared in the journal Proceedings of the National Academy of Sciences.